Recent regulatory developments have led refiners to seek methods for reformulating motor gasolines to meet increasingly stringent air quality requirements. These techniques include reducing the olefin and aromatic content of the motor gasoline while maintaining the desired octane rating by increasing the relative content of isooctane (trimethylpentane) and other octane-enhancing additives such as oxygenates.
Commercial isobutane-butene alkylation, catalyzed by a strong mineral acid such as HF or H.sub.2 SO.sub.4, produces a highly desirable motor gasoline blending component which is enriched in high-octane trimethylpentane. Thus with the advent of more restrictive air quality regulations, the known commercial isobutane-butene alkylation processes present a seemingly ideal solution to the problem of reformulating motor gasoline to minimize both evaporative losses from storage as well as pollutants emissions from gasoline engine operations.
Alkylation is a reaction in which an alkyl group is added to an organic molecule. Thus an isoparaffin can be reacted with an olefin to provide an isoparaffin of higher molecular weight. Industrially, the concept depends on the reaction of a C.sub.2 to C.sub.5 olefin with isobutane in the presence of an acidic catalyst producing a so-called alkylate. This alkylate is a valuable blending component in the manufacture of gasolines due not only to its high octane rating but also to its sensitivity to octane-enhancing additives.
Industrial alkylation processes have historically used large volumes of liquid Bronsted acid catalysts such as hydrofluoric or sulfuric acid under relatively low temperature conditions. Acid strength is preferably maintained at 88 to 94 weight percent by the continuous addition of fresh acid and the continuous withdrawal of spent acid. Liquid acid catalyzed isoparaffin-olefin alkylation processes share inherent drawbacks including environmental and safety concerns, acid consumption, and sludge disposal. For a general discussion of sulfuric acid alkylation, see the series of three articles by L.F. Albright et al., "Alkylation of Isobutane with C.sub.4 Olefins," 27 Ind. Eng. Chem. Res., 381-397, (1988). For a survey of hydrofluoric acid catalyzed alkylation, see 1 Handbook of Petroleum Refining Processes 23-28 (R.A. Meyers, ed., 1986).
Both sulfuric acid and hydrofluoric acid alkylation share inherent drawbacks including environmental and safety concerns, acid consumption, and sludge disposal. Research efforts have been directed to developing alkylation catalysts which are equally as effective as sulfuric or hydrofluoric acids but which avoid many of the problems associated with these two acids, and alternatives such as Lewis acids, e.g., BF.sub.3, have been explored. While Lewis acids generally pose fewer and less severe safety and environmental concerns than strong liquid acids such as HF and H.sub.2 SO.sub.4, it would be desirable to produce paraffin-rich product streams useful as gasoline blending components without the use of noxious and/or corrosive liquid catalyst systems.
Allowed U.S. application Ser. No. 07/883,684, filed Feb. 11, 1992 now U.S. Pat. No. 5,191,150 discloses a method for decreasing the cloud-forming tendency of HF comprising adding a controlled amount of sulfolane to the HF. The application further discloses that conjunct polymeric byproducts (also referred to as acid soluble oil or ASO) accumulate in the liquid acid catalyst mixture and must be removed. The ASO, a complex mixture of polymers, is difficult to separate from mixtures of sulfolane and HF because the ASO contains components having boiling points which bracket the boiling point of sulfolane. The process solves this purification problem by removing (stripping) HF from the mixture to provide an intermediate stream containing less than about 30 weight percent HF. Below about 30 weight percent HF, the sulfolane/ASO mixture readily separates into a less-dense ASO-enriched liquid phase and a more-dense sulfolane-enriched phase. The gravitational separation step depends upon effective upstream HF removal. Because the known processes for removing HF from the ASO/sulfolane mixture (e.g., stripping and fractional distillation) are subject to process upsets, it would be beneficial to provide a separation method which in independent of upstream operating variations. Further, it would be desirable to generate a purified sulfolane stream which can be recycled directly to an operating alkylation reactor without further processing.